1. Field of the Invention
The present invention relates to circulating fluidized bed (CFB) gas distributors and more particularly to systems for gas distribution and bed stability in circulating dry scrubber (CDS).
2. Description of Related Art
The concept of circulating dry scrubbing is well known in the art. See, for example, Neathery, J. K., “A Fundamental Study of Circulating Bed Absorption for Flue Gas Desulfurization,” Ph.D. Dissertation, University of Kentucky, 1993. CDS technology has many advantages over other systems such as limestone wet flue-gas desulfurization (FGD) and lime-based spray drying absorption (SDA). Among the most appealing benefits of CDS are: low capital costs, small footprint, simple construction with few moving parts, carbon steel construction, the absence of a liquid blowdown stream to be treated, and the production of a dry spent sorbent product.
CDS technology incorporates dry sorbent recirculation in a lean-phase transport reactor to achieve sulfur capture. Hydrated lime and humidification water are injected into the bottom of a reaction chamber concurrently with flue gas. The flue gas suspends, dries, and transports the sorbent through the reaction vessel and out into a particulate collector. A large portion of both the spent and unutilized sorbent streams are recycled into the reactor vessel as a dry powder. The recycle of sorbent, from both within the reactor and via the particulate control device, improves the sorbent utilization over other semi-dry methods such as SDA. However, since the flue gas is to remain several degrees above the wet bulb or saturation temperature, the liquid phase coverage of the recycled solids quickly evaporates due to the excellent mass transfer and the abundant surface area available in the riser section.
The venturi section of the CDS reactor acts as a flue gas distributor by ensuring that the influent flue gas is uniformly disbursed across the cross-section area of the riser, as well as maintaining a stable bed fluidization. The use of traditional distributor types, such as perforated plates, bubble caps, spargers, or conical grids for the conditions involved with Flue Gas Desulfurization (FGD) results in plugging which, in turn, causes bed instabilities such as solids drop-out (“sifting” or “weeping”) or gas laning. Venturi distributors allow for a lower operating differential pressure and decreased erosion. Conventional CDS reactors utilize either single or multiple venturi arrangements, depending on the amount of gas to be handled. Most common multiple venturi arrangement includes one central venturi with six circumferentially located venturis.
Besides the number of venturis, the major differences between the single and multiple venturi distributor designs are the installed height required for uniformly disbursing the influent flue gas prior to the gas entering the CDS reactor and the ratio of distributor to bed pressure drop for ensuring bed stability. For conventional venturi-type distributors, multiple venturi designs can reduce distributor height by as much as three times as compared to a single venturi design. However, the height savings comes at the cost of increased venturi pressure drop required to ensure bed stability, such that as much as five times greater operating pressure drop can be required as compared to single venturi designs. As the operating costs associated with pressure drop typically greatly outweigh capital costs associated with reactor height (i.e. reactor and structural support materials), single venturi distributors are often utilized, but without the improved height requirements associated with multiple distributors.
Such conventional distributor systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for systems with improved pressure drop and height requirements. There also remains a need in the art for such systems and methods that are easy to make and use. The present disclosure provides a solution for these problems as well as significant improvements in other criteria that describe a hydrodynamically effective CDS system.